Stage control... In terms of what?
Just like being able to have control over where your opponent goes and such? If in that sense, I say having your opponent above you is generally the best way of assuring that you keep control over their motions and such. Ness' DashGrab, UAir, DashAttack, and PK Fire all do well to cover people attempting to land, UAir being one of the most potent of moves. Having your opponent in the air is almost always the best option. A couple characters defy that, such as MK, Marth, and some other character I'm forgetting.
If you mean in terms of moving Ness around the stage in the most efficient manner, I say learn to AC DAir's everywhere and anywhere. A well-Auto-canceled DAir will be lagless, allowing for you to shield, dash, walk, or jump again with ease. When I feel like messing around a bit in the middle of a match, I just throw off a bunch of AC'd DAir's while moving between platforms to gain some sort of vantage over my opponent. The more easily you can transition between air and ground, and the more easily you can transition between dashing and being in the air (USING UAIR!!!!), the better time you'll have moving about the stage to catch up with an opponent both safely and awesomely.
Remember these things:
BAir has great shield push-back and little landing lag. If you manage to hit the back end of someone's shield, it's normally good. Characters like MK, Marth, and Bowser can often just upB out of shield, but on most other characters, you're safe if you space it right.
UAir has insane range. If you master that hitbox, your opponent will hate you with a passion, because they will both have a difficult time landing and get caught in UAir traps a lot.
DAir AC'd correctly is essentially lagless. Use this to your advantage, but don't over-do it.
WHEN IN DOUBT, SLOW DOWN AND WALK. This I learned from Yink, and it actually helps.
LEARN PK JUMP! It's becoming apparent to me how potent it is against airborne opponents. On people who are on the ground, you're better off with blank jumps and slowly approaching them with like FH stuff (either blank or dropdown AC'd DAir.... or BAir... or UAir.... idk)